In construction of field assemblies for electric machines, magnets must be retained on a cylinder of the field assembly. For example, in a brush type of motor, magnets must typically be retained on a stator housing or a separate flux ring within the stator housing. Ordinarily, these magnets have been glued or adhered to the metallic flux ring or stator housing. This typically involves gluing or adhering each individual magnet to the flux ring or stator housing.
In adhering the magnets to the metallic surface so that the magnets do not shift during use, various types of adhesives have been used. While some of the adhesives have been satisfactory, some adhesives work better than others. As the adhesives age, it is possible that if the device using the electric machine, such as a power tool having a motor, is dropped, that the sudden shock will destroy the bond between the magnet and the housing or flux ring, enabling the magnet to travel within the motor. The magnet itself could also break. When this occurs, the motor ceases to function.
More recently, due to the advent of molded magnets, it is possible to provide an anchor in the flux ring or stator housing and injection mold the magnetic material on the flux ring or stator housing and around the anchor, which then retains the molded magnet material on the flux ring or stator housing. Such an anchor system is disclosed in U.S. application Ser. No. 09/492,059 filed Jan. 27, 2000 entitled Anchoring System for Injection Molded Magnets on a Flux Ring or Motor Housing and in U.S. application Ser. No. 09/764,004 filed Jan. 17, 2001 entitled Anchoring System for Injection Molded Magnets on a Flux Ring or Motor Housing.
However, a disadvantage of the anchoring systems described in the above two referenced patent applications is that they require the use of injection molded magnetic material, which is typically the most expensive type of magnetic material per unit flux.
There are 3 different types of hard magnet materials that are commonly used in small DC motors for portable battery operated power tools: ferrite, bonded Neodymium Iron Boron, and sintered Neodymium Iron Boron. (Neodymium Iron Boron will be referred to herein as “Neo”.) The latter two materials are considered high magnetic energy density hard magnetic materials. (“Hard magnetic material” is material that can be permanently magnetized. “Soft magnetic material,” on the other hand, is material that carries magnetic flux but that cannot be permanently magnetized.) The bonded Neo magnet material can be injection molded or compression bonded. Compression bonded magnets, such as the magnets available from Magnequench, Inc., 6435 Scatterfield Road, Anderson, IN 46013-9606 under the MQ product designations, come in multiple varieties of increasing total magnetic flux: MQ1, MQ2, and MQ3, which is mainly a result of the amount of processing they receive and at what temperature and pressure they are pressed.
The cost effectiveness of magnets can be measured in magnetic flux (in Gauss) per unit cost (in $). In general, the flux per dollar of the previously mentioned magnet materials is as follows from most expensive to least expensive: injection molded bonded Neo, MQ1 Neo, MQ2 Neo, MQ3 Neo, sintered Neo, and ferrite. The flux per dollar of MQ3 Neo and sintered Neo is very close to that of ferrite.